Folic acid is the newest member of the vitamin B complex, and its therapeutic use in the treatment of pernicious anemia, nutritional macrocytic anemia and the macrocytic anemia of pellagra, pregnancy and sprue is bringing the fields of hematology and nutrition closer together. Folic acid occurs in nature in a free form and also as a part of various complexes. The following five substances have been isolated in crystalline form: (1) vitamin Bc; (2) Lactobacillus casei factor from liver; (3) L. casei factor from yeast; (4) another L. casei factor isolated from a fermentation residue, and (5) vitamin Bc conjugate. Vitamin Bc, the L. casei factor from liver and the L. casei factor from yeast are identical with the synthetic product described by Angier and associates.1 This compound is called pteroylglutamic acid and contains 1 molecule of glutamic acid. In contrast, the special L. casei factor